1. Field of the Invention
The present invention relates to tintable, abrasion resistant coatings for solid substrates. The substrates are generally substantially transparent to at least some wavelengths of visible light and either at least partly transparent to ultraviolet radiation or deleteriously affected by ultraviolet radiation. The cured coatings of the invention simultaneously resist abrasion and provide superior tintability. In a preferred embodiment, the cured coating may also block substantially all incident ultraviolet radiation from reaching or passing through the substrate, thereby protecting the substrate and items behind the substrate from ultraviolet radiation.
2. Description of Related Art
Many solid substrates, especially clear, transparent plastic materials, have been used as substitutes for glass in many applications. The reasons for this substitution are the unique properties of the plastics such as weight, ease of handling, and ease of formation of articles.
Plastic materials are not without shortcomings, however, as most plastics are soft and scratch quite readily, and may be degraded by ultraviolet light. To enable the use of plastics in some applications, it has been customary to coat the plastics with organic or siloxane coatings. These coatings preferably are clear, unpigmented coatings. An example of such an organic coating is a heat curable polyurethane resin. Because polyurethane coatings are less expensive, they are considered to be acceptable for coating plastic materials to render them abrasion resistant even though the abrasion resistance of polyurethane coatings is not as good as some siloxane-based coatings.
In order to provide exceptionally hard abrasion resistant coatings to the plastic article manufacturers, new siloxane-based curable resin systems were developed. An example of such a resin system can be found in U.S. Pat. No. 3,986,997. These siloxane resins have been very successful as coatings for plastic lenses, sheets, and other articles. These coatings, however, have two major drawbacks. After curing, they are not tintable and they are transparent to ultraviolet radiation.
It is often desirable for an end application of plastic materials that the abrasion resistant coatings applied thereto be tintable. Such uses, for example, include plastic sunglass lenses. It would be highly desirable, therefore, to find a method for tinting known resins, or to develop a new resin system in which the cured coating is tintable and, at the same time, provides excellent abrasion resistant properties such as that found in siloxane-based coatings.
Others have attempted to produce tintable, abrasion resistant coatings with limited success. For example, U.S. Pat. No. 4,355,135 to January, is directed to a tintable abrasion resistant coating composition for lenses. The coating compositions were tested by measuring the light transmission through the coated lens after immersion in a dye bath for 5, 15 and 30 minutes. While these compositions are tintable, they do not, however, provide the level or speed of tinting desired in many applications. In addition, although the January patent mentions the possibility that ultraviolet absorbers in general can be an "additive" to the abrasion resistant coating, the patent does not suggest that such unexpected and surprisingly improved tintability can be obtained from the coating compositions of the present invention, nor does it suggest the excellent ultraviolet absorbance that can be achieved by the present invention.
Ultraviolet or "UV" light has long been known to have a deleterious effect on certain transparent plastic materials and has also been known to degrade such items as furniture, drapes, upholstery, tapestries, paintings and other artwork, and automobile interiors. For example, ultraviolet radiation from sunlight passing through windows can often damage curtains, drapes, upholstery and other interior or indoor items in a house or automobile. Since light transmission is a desired part of a window's function, any attempt to block or attenuate all incident light, including visible light, is generally unwanted.
Similarly, in order to protect such items as fine art from the deleterious effects of ultraviolet radiation, they are often kept in a darkened or dimly-lit environment and protected by a covering sheet of glass or plastic. This covering sheet may be coated with a compound that absorbs ultraviolet light or an ultraviolet light absorbing compound may be incorporated into the covering sheet. Unfortunately, the aesthetics of the underlying product may suffer from scratches in the surface of the covering sheet and from the darkened environment.
There also has been concern in the ophthalmics industry about the effect of "UV-A" solar radiation, i.e., from 340-400 nm, as a cause of cataracts. Various strategies have been used to provide screening in the UV-A region for eyeglass lenses, whether they are glass or plastic. Darkened sunglasses cause the pupil to dilate in response to the lowered light. This makes the eye more susceptible to radiation at all wavelengths. The presence of an effective UV screen for the UV-A region in an ophthalmic coating, coupled with high abrasion resistance and tintability, would be highly desirable since it would provide protection of the eyes as well as other desirable properties.
One possible solution to the problem of obtaining both a high degree of UV absorption and abrasion resistance is to incorporate a large amount of a UV absorber into an abrasion resistant coating composition. Unfortunately, the low compatibility of UV absorber compounds in abrasion resistant coating compositions can lead to loss of adhesion, transparency, or abrasion resistance, and has prevented the development of a single composition that has both high abrasion resistance and excellent UV absorption at a reasonable cost.
Accordingly, the need exists for a single coating composition that, when cured, can accomplish the objectives of high tintability, combined with significant resistance to abrasion and, optionally, the ability to absorb ultraviolet radiation.